A tensor invariant is a polynomial in the tensor \(T\)  and its conjugate \(\bar{T}\)  which is invariant under the action of the tensor product of
\(d\)  independent copies of the unitary group \(U(N)\) . The sum is taken over a finite set \(\mathcal {B}\)  of such invariants \(d\) -bubbles [1]} associated with the couplings \(\lambda _b\) .
